Technical Abstract:
As the US swine industry moves towards group housing sows, it is important to increase our understanding of aggression. Sows fight at mixing and this project sought to determine the amount and type of aggression observed when unacquainted York × Landrace sows were mixed in pairs or in two established sub-groups of three. Treatment 1 (PR) used 11 pairs of sows mixed into a 6.2 m**2 pen. Treatment 2 (GP) used 14 unacquainted groups of 3 sows, with 2 groups mixed into a 19.2 m**2 pen. Behavior was recorded continuously for 60 min post-mixing and all-occurrences sampling was used to extract social interactions. The data were analyzed to determine the number of social interactions that did or did not contain aggressive components (i.e. pushing, knocking or biting) and fighting defined as interactions that contained 10 or more reciprocated, aggressive component actions. Within each interaction, data were analyzed to determine the number of component actions and the number of aggressive component actions. The data were compared using a GLM, with treatment as a fixed effect. The number of social interactions was higher in groups (54.3 ± 3.7) than in pairs (20.9 ± 2.9, P<0.001). The number of interactions that contained aggression was similar (15.4 ± 1.8 v. 10.3 ± 1.5, P>0.05), but GP interactions contained more aggressive components (26.4 ± 2.4) than PR interactions (6.8 ± 0.4, P<0.001). All PR pairs fought and aggression escalated from pushes, to knocks, to bites. In GR mixing, 13.7 ± 0.4 of the 15 possible pair combinations interacted socially, of which 8.1 ± 0.8 interacted aggressively and 4.0 ± 0.4 fought. A higher proportion of unacquainted pairs engaged in aggressive interactions (0.71 ± 0.11) and fighting (0.43 ± 0.04) than acquainted pairs (0.29 ± 0.07 and 0.02 ± 0.02 respectively, both P<0.05). Bites were delivered first, then knocks, then pushes. The results indicate that sows mixed in pairs appear more circumspect, but that escalation and fighting appear inevitable, whereas in group mixing, aggression is more immediate, but not universal. The results further our understanding of aggression at mixing and will help us to determine best practice for producers.